The quake was felt over all of Wales, most of western England, as far east as London and as far north as the southern edge of the Lake District, the British Geological Survey (BGS) said.
The epicenter was approximately 20km north-north-east of Swansea and at a depth of 7.4km, it added.
There have been no reports of serious damage or injuries.
Events of this magnitude happen in the UK every three to five years, the BGS said.
Roger Musson, from the BGS, said it was the largest earthquake in Britain for the last 10 years.
He said it was "relatively shallow" for a larger British earthquake. They are usually around 15-20km deep.
'The walls cracked'
Vera Sanderberg, from Croyde in North Devon, said she felt her house shake.
"The neighbor's horses bolted," she said.
"It was just a split second. We can't see anything damaged."
In Cheltenham, Paul Samway said it felt like "a washing machine breaking down".
"We heard the walls crack and there was a bit of juddering," he added.
"We thought nothing of it until we saw reports from other people on social media."
Louise Craig, from Connah's Quay in North Wales, said it was "the most excitement I've had on a Saturday afternoon for a while".
"I get a lot of fat wood pigeons jumping up and down on my chimney and I, first of all, thought the noise was them but they must have put on a heck a lot of weight to make the house shake," she told the BBC.
Donna Johnson in Caerphilly said she was "really scared".
"The house started rocking, and I said, 'oh my god the house is falling down'. We have a bungalow, and there is a risk of subsidence here," she told the BBC.
"I ran outside - I was shaking like a leaf... The cat went outside before me, so she must have known."
Sue Field in Shifnal, Shropshire, said she felt the tremors from her armchair.
"I thought I'd had one too many wines for my birthday lunch," she said.
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